Discrete resistance trimming for electrical resistance strain gages is known. Known techniques for discrete trimming strain gages do not attempt to establish a uniform grid layout that faithfully tracks an applied strain field and indicates an applied peak strain. U.S. Pat. Nos. 2,885,524 and 5,227,760, and Japanese patent documents 2006-234384 disclose strain gages including discrete trimming without regard to the strain field measured.
FIG. 1 illustrated a known linear strain gage 100 with a sinuous base grid 102 that serves two functions: 1) to set the initial resistance; and, 2) to sense the strain field applied beneath it (e.g., from the specimen to which it is attached).
For convenient operation, electrical resistance strain gages must have a resistance value trimmed to a close tolerance. Strain gage grid 102 can be adjusted for electrical resistance by mechanical thinning of the metal alloy from which the strain gage is produced, or by using additional, discrete resistance elements which are introduced to, or removed from, the grid circuit via a trimming operation. The discrete resistance provided by the trimming elements introduced or removed is sometimes referred to as the trim resistance. The resistance of the linear strain gage 100 will be referred to as bulk resistance in this application to distinguish it from the trim resistance.
Electrical strain gages are physical integrators. The grid 102 responds to an applied strain field by changing resistance in such a manner as to average the applied strain magnitude over the active measuring area 108 defined by the grid length 104 and the grid width 106. In its simplest form, this relationship is expressed as:ε=ΔR/R/F  (1)
where:
ε=applied strain
ΔR=change in grid resistance
R=initial, unstrained grid resistance
F=transfer coefficient, typically called gage factor
This relationship is deceptively simple, and presumes a uniform displacement field applied beneath the grid 102. In reality, the applied displacement field is often a non-uniform function, which the grid will integrate to find the average value impressed upon the active measuring area 108 (grid length 104 X grid width 106).
The plot 200 in FIG. 2 illustrates a representative prior art plot of strain 202 vs. position X along the length 104 of the strain gage 100 to demonstrate this phenomenon. An exponentially varying strain function is impressed upon the strain gage active measuring area 108. The active grid resistance (the resistance of the active measuring area 108) is changed by the applied strain such that the function is integrated over the active measuring area 108 and the change in grid resistance is proportional to the average magnitude 204 of the applied strain function, averaged over the active measuring area 108.
This integrated value of strain can be much lower than the highest peak value 206 present under the grid. For those applications concerned with structural design, the highest peak value 206 is normally of most interest, because that is the strain magnitude which will cause specimen failure. A measured strain reported lower than peak value can compromise the analysis, possibly resulting in an unsafe structural design.
For those applications concerned with commercial weighing, peak output from the strain gages allows optimum fatigue design and maximum resolution from the weighing transducer. If the strain gage design in some way reduces the possible transducer output, then either the transducer must be designed with higher stress levels (compromising load-cycle life), or the lower output accepted (compromising weighing resolution).
Although the main grid area 108 (e.g., the pattern of contiguous sinuous lines) is the primary sensing zone for the strain gage 100, every contiguous resistance connected between the solder tabs 112 (FIG. 1) for lead wires (not shown) and in contact with the strain field will change proportionally to the applied strain and the initial infinitesimal value of resistance along the path will change proportionally. Therefore, any and all resistance connected between the solder tabs 112 will contribute to the overall resistance change of the gage 100, which is sensed by the instrumentation (not shown) electrically connected to the solder tabs, and becomes part of the overall reported average strain value (e.g., 204). In practice, the ancillary resistance outside the active measuring area 108 is minimized, so as to define the measurement zone as effectively being only the active measuring area 108.
In prior art discrete trimming strain gages, the trim resistance is often introduced to the measuring area (defined by grid length 104 and grid width 106) using partial-length grid lines 114, sometimes with varying angles to the principal measuring direction, and also using additional trimming-resistance areas outside of the primary measuring area, often at varying angles to the primary measuring direction.
Achieving a more accurate measurement of applied peak strain from an electrical resistance strain gage, a measuring area 108 consisting of uniform length grid lines 110 and containing all trim resistance, comprising trimming resistance elements aligned in the principal measuring direction, is desired. The present invention achieves this goal by incorporating the trim resistance with the bulk grid resistance, as opposed to locating the trim resistance outside the active grid area and/or introducing measuring grid lines of varying length to provide required resistance trim steps.